Members that constitute a vehicle body need different strengths and rigidities depending on the portion. For example, left and right side frames are required to be able to deform at preset positions and effectively absorb collision energy when the vehicle receives a collision load. To implement this, generally, the material (for example, the properties of the material or the plate thickness) of the left and right side frames is changed depending on the portion, or the left and right side frames are partially reinforced by reinforcing members. However, this increases the number of members constituting the left and right side frames and also serves as a factor to increase the cost, and the weight of the vehicle body increases.
In recent years, a technique of forming members that constitute a vehicle body, for example, left and right side frames by hot-stamping (hot-pressing) a steel plate is used. In addition, a technique of changing the quenching hardness for each predetermined portion has been developed. These techniques are known by, for example, PTL 1 and PLT 2.
In the technique known by PTL 1, a heated steel plate is press-molded, thereby obtaining a vehicle body member such as a side frame. Additionally, in the technique known by PTL 1, when press-molding the heated steel plate, the cooling speed is partially made lower than in other portions, thereby providing a plurality of fragile portions for which a lower hardness is set.
In the technique known by PTL 2, left and right crush rails are provided at the front ends of left and right front side frames. The left and right crush rails are rectangular closed section bodies extending from the left and right front side frames to the front side. Each of the left and right crush rails is partially provided with at least one crush trigger. The left and right crush triggers are fragile portions obtained by heating and cooling the left and right crush rails at predetermined positions. If the front of the vehicle receives a collision load, the left and right crush triggers can control the deformation positions of the left and right crush rails.
In general, the left and right side frames are members that are located on both sides in the vehicle width direction and are long in the vehicle longitudinal direction. One end of each of the left and right side frames is the end, that is, the distal end on the external force acting side that receives a collision load in the vehicle longitudinal direction acting from outside of the vehicle. The other end of each of the left and right side frames is the end, that is, the proximal end on the opposite side of the end on the external force acting side.
The proximal ends are joined to the central portion of the vehicle body, for example, the ends of left and right side sills. The left and right side frames have a so-called cantilever structure with the fixed proximal ends. If the distal ends receive a collision load in the vehicle longitudinal direction, a bending moment can act on the left and right side frames. The magnitude of the bending moment changes depending on the height of the point of action of the collision load to the distal end. The bending moment at the proximal end is larger than the bending moment at the distal end.
To effectively absorb collision energy by the left and right side frames, they preferably simultaneously deform as a whole. In the techniques known by PTL 1 and PTL 2, however, there is room for improvement to effectively absorb the collision energy.